Attenuator circuits find wide usage in RF, UHF and microwave circuits. A common device of these attenuator circuits is a PIN diode. The PIN diode is fundamentally a device whose impedance at RF, UHF and microwave frequencies is controlled by a DC current through the PIN diode. The PIN diode is a semiconductor diode in which a high resistivity intrinsic I-region is sandwiched between a P-type and a N-type region. When the PIN diode is forward biased, holes and electrons are injected into the I-region. These charges do not immediately annihilate each other; Instead they stay alive for an average time called the carrier lifetime. This results in a an average stored charge which lowers the effective resistance of the I-region. Hence, the PIN diod functions as a voltage controlled resistor at said frequencies.
A PIN diode attenuator is a device that provides a predetermined value of attenuation in response to a precise stimulus, for example a voltage bias.
A classical implementation of a conventional PIN diode attenuator comprises a quadrature hybrid which is shown in FIG. 1. This conventional PIN diode attenuator provides matching impedance at all levels of attenuation and high linearity. However, this conventional PIN diode attenuator is very sensitive to intrinsic PIN diode fluctuations due to the bias voltage precision at high levels of attenuation. Hence, the notch point of the conventional PIN diode attenuator fluctuates due to intrinsic PIN diode variations. This results in limited repeatability of manufactured PIN diode attenuators. Calibration by means of generating look up tables for bias voltage versus the corresponding attenuation of the manufactured conventional PIN diode attenuator is required.
In order to improve the conventional PIN diode attenuator several solutions have been earlier suggested.
In US6,973,288B1 is a linearizer circuit disclosed. In this solution two attenuator control inputs are used, causing the complexity of the attenuator to increase. The basic function of this solution is to provide a linear relation between control signal and attenuation. A more severe drawback of this solution is that the attenuation will be sensitive to bias level due to the exponential relation between voltage and current in a PIN diode.
US20070001738 discloses a variable resistance circuit. In this solution the complexity is increased compared to the above mentioned solution. This solution utilizes an adder circuit to sum two signals from two different circuits comprising voltage sources and a zener diode. This solution also attempts to linearize the response from the variable resistance circuit. This solution also exhibits the earlier mentioned drawbacks of being sensitive to changes in the bias voltage. In addition some additional drawbacks are added due to temperature drift and manufacturing repeatability for the zener diode.
US6,091,299 discloses yet another solution aimed to linearize the response of a PIN diode attenuator by using an array of zener diodes. The array of zener diodes is used to generate a linear response. This circuit also requires a large amount of trimming in order to obtain a linear response, the complexity of the circuit is also increased.
US7,023,294 discloses a solution for linearizing a PIN diode network by means of a complex circuitry comprising several parallel resonant circuits causing a cancelling of unwanted parasitic reactance.
The known solutions in the art have drawbacks, which cause problems when put into operation.